What is an active centre of a catalyst? I was studying catalytic poisoning and read that in temporary poisoning, the poisoners are held at active centres by weak forces.

What is an 'active centre' in this context?
The active centre is the site where the substrate binds to the catalyst in order for catalysis to take place. On a molecular level, most catalysts (or most substances really) are not just flat surfaces, they have complex three dimensional shapes. The substrate often has to bind to the catalyst in a very specific way in order for the catalysed reaction to take place - this is the active centre.