Players commonly say something like "combat?" or "attackers?" which is basically a short way of asking "I am ready to declare attackers, do you have any spells or abilities you want to use in the beginning of combat step?" This is your last chance to tap anything. If you say no then they can proceed with declaring attackers, which is a so called "turn based action" that you cannot interrupt.
If a player just starts turning things sideways and declaring attacks without giving you a chance to act, you can ask them to please rewind to the Beginning of Combat Step. However, you are not allowed to "fish" for information about their intended attackers and then try to use abilities.
A creature becomes an 'Attacking Creature' as soon as it's declared an attacker, so you can target it with "target attacking creature" spells/abilities anytime from before declaring blockers up to End of Combat before the second main phase starts.
Likewise a declared blocker is a valid target for 'Blocking Creature' spells/abilities before or after combat damage, until the second main phase starts.
This sometimes catches new players off guard with a card like Celestial Flare since if you attack with say, two weak creatures and a strong one, I could kill your two weak creatures with blockers then cast Celestial Flare during the End of Combat Step and force you to sacrifice your strong creature