When I was a baby, I didn't understand what it meant to take oxygen into my lungs to be absorbed into my bloodstream. But that didn't make breathing any less meaningful to me.
Plan9 has a good explanation. Baptism is a sacrament, which means it is about what God is doing/has done for us, not about what we are doing or did. Holy Communion is not about my getting up and coming to receive, it's about God calling me to come receive what He has done for me.
Infant Baptism is about God's Prevenient Grace (see my sermon on the Nazarene Article of Faith
http://christianforums.com/t724064). Prevenient Grace is that grace of God which goes before. The fact that you are aware of sin, the fact that you are able to have faith, the fact that you are able to repent are all evidences of God's Prevenient Grace at work in your life.
Sacraments are the ordinary (not only) means by which God's Grace is mediated to us...prevenient, justifying, sanctifying, and sustaining grace...and so we understand infant baptism to be a mediation of God's prevenient grace.
That is, at least, one Nazarene's perspective. It may or may not be entirely in harmony with the teaching of my Methodist brethren.
Grace and Peace,
WJ