Evaporation is when a liquid becomes a gas without forming bubbles inside the liquid volume. If bubbles are formed we are talking instead about "boiling".
For example, water left in a bowl will slowly disappear. The water evaporates into water vapor, the gas phase of water. The water vapor mixes with the air.
The reverse of evaporation is condensation.
When the molecules in a liquid are heated, they move faster. This makes them full of energy and so the particles collide with each other, and eventually they become so far apart that they become a gas.
During evaporation only the molecules near the liquid surface are changing from liquid to vapor. During boiling the molecules inside the volume of the liquid are also changing to vapour. For this reason during evaporation no bubbles are formed, instead they are formed during boiling.
Evaporation can happen at any temperature, while boiling happens only at a specified temperature called the "boiling point". Evaporation happens slowly, but boiling happens quickly.

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