Depends on the implementation, and I think that that's something of an issue in discussion about it -- because its effects depend a great deal on the specifics.
There is a portion of the small government conservative crowd that sees it as a replacement for welfare programs where the government mandates policy. Like, instead of getting, oh, food stamps or such, where the government precisely spells out policy in each area ("this is what you are permitted to buy with this"), people who are poorer than a certain amount would simply get a flat cash payment and choose how to use it. In that sense, it'd reduce the degree of control that government has, which is a goal that they'd like to see.
There's also a portion of the redistribute-more-wealth crowd on the left that sees it as existing alongside existing welfare programs, rather than as a replacement. For them, if the government has progressive taxation policy (like, income tax brackets or the like), a flat benefit to everyone will tend to redistribute more, which is a goal that they'd like to see.
Both implementations would qualify as UBI -- they both provide an unconditional basic income. But the actual effects depend on the implementation.
So when someone says something like "sign this petition for UBI", I think that a really good question is "tell me what sort of UBI you are aiming to have implemented", because the details have a very considerable impact on what it is that you're signing up to support.