Onegaishimasu means 'please', it's formed from the verb negau- to wish. The full expression used at the start of a game is yoroshiku onegaishimasu, yoroshiku means something like 'well', when two players of widely disparate strength meet only the weeker says "yoroshiku onegaishimasu". Yoroshiku can be written with the numbers 4649 so this is often seen on the internet.