Knowing the strength of your hand

I have heard so many people talking about how important it is to know the strength of you hand - what does that mean?

Does it mean that we should know how our compares to all other possible hands? There's like 1300 possible combinations hwo can we possibly remember that!

Having said that, I started playing around with poker stove and Q6o turns out to be about 51% against a random hand - who'd a thunk! So I do realise the importance of knowing the strength of my hand but I will really appreciate it if someone would tell how to go about getting this info.