Texture refers to the overall sound of a piece of music. It can be thick or thin, busy or sparse and so on.
Texture is determined by how many instruments are playing, how many different parts there are and the timbre of the instruments playing.
This beautiful melody on flute has a thin texture as one line is being played by one instrument.
Storm Interlude (from Peter Grimes)- Benjamin Britten
This piece of music has a very thick texture because there are lots of different parts or musical ideas playing at the same time and many different instruments with contrasting timbres.
Listen below. Beware, it is hectic and exciting...