On a normal turn, a player has Movement, an Action, and a possible Bonus Action. Players also have a possible Reaction that can take effect on your turn or on another character's turn.
Movement allows you to move up to your speed, and you can also break up your move around your actions, eg move 10', shoot a crossbow, move 20'.
Actions can be one of the following: Attack, Cast Spell, Dash, Disengage, Dodge, Help, Hide, Ready, Use Object, or something else that the DM determines might take up the entire round like searching through a bookshelf.
Bonus Actions are specified by an ability or skill. 2nd-level rogues, for example, can use a Bonus Action to Dash, Disengage, or Hide. The monk can use a Bonus Action to make an unarmed attack IF they take the Attack action with a monk weapon or unarmed strike.
Reactions are an instant response to some kind of trigger. For example, the Shield spell can be triggered by taking damage; the other character hit you with an attack, so you spend your Reaction to cast Shield to increase your AC and hopefully turn that hit into a miss. You can only take one Reaction until the start of your next turn.
The Ready action is a special case. You basically set a condition or circumstance that will trigger either an action OR movement (not both). You must use your Reaction to accomplish this. So you might say, "I wait until the bandit enters the room, then I will attack them with my sword," or, "If my friend appears at the intersection, I will move 30 feet into the alley." Then your turn basically ends and you wait to see if that condition is met; if it is not met, you don't do anything (NB you can also choose to NOT do what you said you would if the trigger is met).
The tricky part about Ready is that because you're spending a Reaction to do it, sometimes you don't get to perform other things you normally would. Using the Monk as an example, if you take the Ready action to strike the bandit with your fist, you're technically NOT taking the Attack action, which means you can't use your Bonus Action because 1) the rules say it has to follow an Attack action, and 2) BAs are taken on your turn, not someone else's.
The same goes for warriors with Extra Attack making an Opportunity Attack; you're spending a Reaction to make one melee attack, but by rule you don't get to use Extra Attack because you didn't take the Attack action.