With a name parodying Hyde Park, Hide Park is a park in Ankh-Morpork with a small lake. Its unusual name comes from old times, when a hide was the measure of land a man with a bullock could plough on a wet Thursday. That is exactly how big the park is. This hide is much smaller than the old Roundworld measure, which described the area a man could plough and cultivate for the season, often 120 acres. This is again much smaller than Dido's hide, which encompassed the city of Carthage.
The park is neutral territory in Ankh-Morpork. There aren't many places in the city where you can't be robbed and/or killed, but the park is one of them. Even the worst of criminals likes a nice place to have a picnic. Like other such spaces in great cities around the multiverse, Hide Park encourages a class of alfresco orators who harangue and entertain the visitors on serious and frivolous topics. The Patrician enjoys such free expression of ideas: he likes to know what everyone is thinking (and his clerks like a walk in the park, too).
There is a sketch on page 26 of Book:The World of Poo showing the Park as seen from the perspective of someone standing at the Gibbet end of Nonesuch Street, looking Hubwards-by-Widdershins up towards the Tump. The scene shows the scope of the Park, with lake, and the distant panorama of housing and streets climbing the side of a distant hill which by context has to be the Tump, which is bare on top except for a disappointingly small Tower, perhaps the only inconsistency about this sketch. (Although this might be down to an artist working imaginatively from The Streets of Ankh-Morpork, and seeing only "the Ruins" at the top of the Tump - the Tump Tower came later.)