When one thinks of a Seer we tend to imagine someone who gazes into a crystal ball and predicts the future. Oh course, Seers in homestuck are less tacky and more awesome than a woman sitting in scarves on the side of a road (I’m not dissing real life Seers, they’re actually pretty cool, but some of them are super fake and cheesy). Basically a Seer completely and utterly understands their aspect. They are stated as one ‘one who leads and guides with knowledge pertaining to [aspect]’. Seers also appear to have a guideline for SBURB implanted in their mind, and thus are extremely handy to have as they can lead their teammates towards the right path.
Space is the most important aspect to have in a session as without a Space player the Genesis Frog cannot be bred and thus the session is rendered null/void (not sure which term is correct here). Space deals with the physical places around us, and also appears to have something to do with physics (gravity, acceleration, motion, velocity,mass etc). Space players are able to bend the fabric of paradox space to their will, but this power would manifest different depending on what class.
The Seer of Space would have an inherent knowledge of the way the universe works, and can use this to their advantage by guiding their friends towards the outcome which is most successful spacewise. Which I would take to assume means that they will guide their team towards the end which results in the least destruction of Space.
I hope that helped a little bit.