A round is about 6 seconds in DnD 5e. A round consists of each creature involved in the encounter taking their turn. Some things can only be done once per turn, like a rogue’s Sneak Attack feature, while other things can only be done once per round, like taking a reaction.
During a turn, a creature can move, take one action, take one bonus action if it is available, and make one free object interaction (such as unsheathing a sword or closing a door). During another creature’s turn, it can also take a reaction when possible.
Because so much happens on each turn, it can be helpful to think of DnD’s combat as much faster and more seamless than DnD gameplay suggests. For example, if 10 creatures all move around and attack in the space of 5-7 seconds, the action is all happening at once, more or less, with initiative making a fraction-of-a-second difference in the actual game world.
With most DnD fights lasting fewer than 10 rounds, combat in 5e rarely lasts longer than a minute (of in-game time – in real life time, a fight can take over an hour :)).
Player’s Handbook, page 189-90