Blocking is an important first step to defense in fighting games. There are 2 types of blocks in most fighters, standing and crouching. There are 3 block zone attributes that all moves have, high, mid, or low. Standing block will block Mid and High. Crouching Block will block Mid and Low. Almost all jumping attacks hit high. A few crouching attacks usually hit low. Some characters have standing moves that can hit "Overhead", meaning they hit high, even though they're not a jumping attack. As a point of design, Overheads are always slow enough that they can be reacted to, usually 15 frames of startup or more. Low attacks meanwhile can be extremely fast, as fast as 5 frames or less in some cases.
This means moves that can hit you mid or low can be unreactably fast, and moves that hit high require the opponent to jump or use a slow overhead, so they're always reactable. This means by default you should block crouching, until your opponent jumps or does an overhead, then you block standing.
If your opponent jumps in on you, they can immediately follow up with a low attack, so get used to blocking jumping attacks standing, then crouch blocking as soon as they land.
There are a few exceptions, such as tiger knee special moves (doing a special move motion, then pressing up to do it instantly at the lowest possible height in the air), and Fuzzy Guard instant overheads (hitting the opponent's standing block, so that they will continue to stand when they switch to crouch block, allowing you to hit them with an air normal), but these are rare and character-specific.
Be aware that if an opponent jumps past you, they can hit you with a crossup attack that requires you block the opposite direction, so you need to walk forwards under opponents who are doing this.