From SuperCombo Wiki

A link is a common type of combo across fighting games, allowing moves to combo without canceling one into another. A link is when you first use a move that has enough frame advantage to fit the startup of another move in before hitstun ends. You can determine which moves link by looking at a framedata chart and looking for moves that have a lot of frame advantage, like +6, then looking for moves with less startup than 6. You can find all the link combos on your character simply by cross checking between the frame advantage of each move, and the startup of other moves. If the frame advantage is more than the startup of another move, then those moves can link. Links are usually denoted by a comma in standard Combo Notation.

In Street Fighter V, Cammy has s.MP, which is +7. This means it can link into any move with 7 frames of startup or less, which is all of her ground moves, except s.HK and c.HP. Her c.MP is +5 with 5 frames of startup, which means it can link into itself, or any light attack, which each startup in 4 frames, but it can't link into s.HP (6f startup) the same way s.MP can, because it doesn't have enough frame advantage.

Another factor to be careful of in link combos is making sure pushback doesn't push the opponent too far for the next move to reach. In the example above, Cammy's s.MP cannot link into itself because it pushes too far away. In Capcom vs SNK 2, Cammy's cl.HP is +10 with 3 frames of startup, but it pushes the opponent so far away that it cannot combo into itself. The only move that will reach at that distance is c.MK.

By timing attacks so they are Meaty (hit on the last active frame) you can get more frame advantage, making new link combos possible. Cammy's c.MP (+5) can't normally link into s.HP (6f startup) in the example above, but it has 3 active frames. If you time a meaty attack so that the last active frame connects, c.MP will become +7, 2 more frames of advantage over its normal +5 advantage, because you used the 3rd active frame instead of the 1st active frame. This means it can now link into s.HP.

Counter hits can also grant more frame advantage in some games. In Street Fighter V they grant 2 extra frames of advantage. Similar to Meaty Attacks, this can cause moves to link that wouldn't normally. For example, if Cammy gets a counterhit on her c.MP, it's now +7, meaning it can combo into c.MK (7f startup). This means Cammy can do the blockstring, c.MP, c.MP, c.MK, and if the opponent tries to attack between the two c.MPs, they will get counterhit, allowing the second c.MP to combo into c.MK, functioning as a hit confirm for a special cancel off c.MK. In short, Counter Hits can link together moves in blockstrings that otherwise wouldn't combo, allowing more sequences of moves to function as blockstrings than just moves that link into one another on normal hit.