Aikido literally translates as "the way of harmony with ki". Unfortunately the concept of "ki" does not have an exact western equivalent but probably the nearest we are going to get is "spirit" or "life force".
The founder of Aikido, Morehei Ueshiba, was perhaps the most formidable martial artist of his day but he was also - and this is not a strange combination in Japanses culture - widely regarded as one of the most spiritual men of his time.
Rather than spend a lot of time re-iterating what is on lots of other Aikido-related web sites I will point you to the excellent Aikido Primer written by Eric Sotnak which covers all the basics of Aikido. Although there may be parts of this article which do not apply to the Fudokai Aikido Dojo and its principles, it is a very good general introduction to Aikido.
A copy of the Aikido Primer is available for download as a PDF document.