When we talk about texture in music, we are basically talking about how much is happening in the music at any given time, in terms of how many instruments/voices are playing or singing, whether they are playing the same or different music, and so on. At GCSE, it is good to know a few technical terms to describe texture, as this will get more marks than basic terms such as 'thick' or 'thin'. Good terms to use are monophony, homophony, melody-and-accompaniment (also known as melody-dominated homophony), and polyphony (also known as counterpoint).
Monophonic music has a single line of music being played or sung as a solo, or in multiple parts but in unison (i.e. at the same pitch) or an octave apart. Homophony means that multiple parts are moving together in some sort of harmony; this is often just chords moving together, such as in a hymm. Melody-and-accompaniment means, as the name suggests, a tune over an accompaniment, for example someone singing a solo over piano chords. Polyphonic, or contrapuntal, music has two or more different tunes being played/sung at the same time, or the same tune being started at different times in different vocal/instrumental parts so that at any given time each part is singing a different part of the tune – for example when a group of singers sing a round, or canon. It's easy to find examples of each type of texture using YouTube, and it's always best to learn these things by listening rather than just reading. There are also other interesting textures such as antiphony and heterophony which you could explore.