The difference in the structures of pyrrolidine and pryrrole results in a difference basicity of the two compounds. In this case, we are looking at Lewis acid-base strengths which result from electron pair acceptance for acids and electron pair donation for bases. While their general structure (pentagon) is the same, pyrrolidine does not have any double bonds while pyrrole has them. As a result, pyrrole is an aromatic compound which limits the availability of the lone pair of electrons on the nitrogen for bonding with electrophiles. Since the electrons are less available for bonding, they are weaker electron donators and therefore a weaker base. Pyrrolidine does not have the double bonds and is not aromatic so the lone pair of the N is more available (more easily donated) so it is a strong base.