Default installation of phpBB forum is not search engine friendly. Each post of phpBB forum is accessed with url like
phpBB also add sid (session id) to track the user, this will make the url difficult to get listed in search engines.
With phpBB Search Engine Indexer MOD, you can make your phpBB threads look like HTML pages. Having each thread as HTML with keywords in it make the page get indexed by search engine.
You can see phpBB Search Engine Indexer mod with forcetype installed at