An angle that is smaller than 90 degrees. It looks kinda like an arrowhead or a wedge. It is smaller than a right angle (which is exactly 90 degrees). You can imagine it like a door that is open just a little bit.
An easy way to remember is to imagine a door that is slightly open. It also looks similar to an arrowhead or a wedge. Other familiar shapes are star-like shapes that also use acute angles.
Examples of shapes with an acute angle
Here are some examples of shapes that use acute angles: