a patch is only going to work for you if you are building your own pidgin (read as: you have a maemo developer environment set up). Arguably, if you can get a maemo developer environment set up, you have experience with patch/diff.

However, in brief, you would run the 'patch' command against the source code of maemo-pidgin with the patch file as an argument and it would 'patch' the source of pidgin to supply the functionality you want. You can get a lot more detail by googling "patch" and "diff" together (first few links)

So, to summarize: you cannot 'install' a patch, you have to rebuild from source after applying the patch. The general hope with patches is that they get accepted into the source code and so become 'default' and then will become available in a later update of the software.

Hope this helps explain it.