The flavor and whole 'sensory experience' is made of the interplay between several different components of your mead.
Sweetness - The sweetness or sugar present, usually measured by final gravity (FG); can also be represented in g/L
Acidity - Sour, tart, etc; more complex than just sour
Tannin - Body, richness, and other 'mouthfeel' components; comes from wood, fruit skins, herbs, nuts, some vegetal matter
Alcohol - The alcohol level does affect the overall balance point as well, a stronger ABV mead may require more of the other elements to achieve a balance with the alcohol
Unbalanced meads are going to be 'missing' something. Newly finished meads are often unbalanced across all aspects. They are dry, low in acidity, and often low in tannins. Exceptions will be meads that stalled or fermented to tolerance with residual sugar, ones that either had acid in the ingredients like fruit or acid was added, and ones such as tea or strong spices in them.
[Content expansion planned]
Balancing is best done via bench trials. This will allow you to test a change on a small sample, before committing changes to your entire batch.
Note: this assume your mead is a stable mead
Honey is the usual sweetener for mead, but that's not strictly required. Any form of sugar will work and includes table sugar, brown sugar, molasses, lactose, fruit, malt extract, etc.
Not all sugar sources have the same 'sweetness' perception per gram of sugar/point of gravity. Honey is composed largely of glucose and fructose, simple sugars as opposed to fruits which have slightly complex sugars.
All sugars are also not fermentable. Lactose is the typical example that yeast do not ferment. Some bacteria can, so all proper sanitary methods should be followed. Molasses also contains some unfermentable sugars as well.
Another article has backsweetening information.
[Content expansion planned]
Acids shift the balance from 'flat' to 'brighter'. Acid type has different perception. A balance between acid types also has a particular interplay. Too much of one acid can be balanced by adding a small amount of others. Consider acid blend if finding a good ratio is difficult.
Citric acid - quick and bright
Tartaric acid - 'tart', quite literally where the name comes from
Malic acid - Last longer on the palate than citric, most common acid in apples
Lactic acid - smoother acid, the result of malolactic fermentation (MLA)
Bench trials with acid types should be perform per the guide. Acid increments should be very small as they are potent additions.
Acid blend is a combination of citric, malic, and tartaric in the varying ratios (by brand). These are the main acids found in grapes. It is an effective option as a one-stop-shop for doing acid balance work.
Not all acids are desired. Avoid organic acids such as acetic acid (vinegar). Unlike kombucha, acetic acid is a detrimental flaw and detectable in small amounts (on the order of 1g/L). Additionally, fatty acids are very damaging to the palate as well. They are rare and unusual as an acid addition, but listed for completeness.
[Content expansion planned]
Tannins are commonly added via powdered wine tannins, oak/wood, or other herbs.
Other types of flavor components can be added.
Aroma
Bitterness
Spices (Such as cinnamon and clove)
Spicy (such as heat from peppers)
Here is an example of elements that can shift flavor balance in other ways. Sweetness/acid/tannins is the most common and works for all styles. These are different elements which can shift the experience one way or another, and round out flavors. They are tools which can be considered to rescue a mead.