texture reads

I’ve heard this phrase used a fair amount, but I’ve never really been clear on what it does, or, what it refers to in code - also people prefix it with ‘dependent’, and I don’t understand this either. Conversely - is there an independent texture read.
Finally, what is a texture lookup? is it like a lookup table or something?

I’ve done the odd search but nothing I find seems to give me a conclusive answer.

Any references/explanations would be appreciated.