It's hard to translate this phrase into English, because English has only one form for 'you', regardless of whether one person or number of people are being addressed. In Russian, 'вы' (plural) is used to address a group of people (something like 'you guys'), and 'ты' (singular) to address a single person. Also, Вы is used to address a person in a polite or/and formal way (a stranger, person older than you, business partner etc.). So, if somebody says to you 'давай перейдем на ты' (let's switch to ты) or, as in your example, 'можно и на ты', it means that this person doen't want to be too formal.