You're meant to go the lectures and read the slides in your own time/read around the topic/research things you don't understand. Not one or the other.
If it's just your first year then it probably won't be a big deal, but it's not a good habit to develop. There will be things in the lectures that aren't on the slides, and depending on the lecturer it could be very important things that aren't on the slides (especially in later stages of the degree).
It's also good to have had a good level of interaction with lecturers so that as you get further into your degree it's easier to approach them for help. In my experience academics are extremely unlikely to put in any effort to help you if they don't think you're putting in any effort to help yourself. This is particularly important if you're doing a science degree and you get your third year honours project assigned - if you never show up to lectures and the academics have no concept of who you are then you might end up with useless project.