Some seeds will germinate with just warmth and water but if they do not get sufficient light they become spindly and stretched out. These seedlings are weak stemmed and tough to plant out.
You get better seedlings by growing them under light. Once they are around the two leaf or four leaf stage you can take the top off your tray and move them into indoor sunlight if you wish but if you let them dry out then it's all over! I prefer to keep mine in high humidity and bright artificial light for four to six weeks and then take outside to harden off in some shade.
From a technical point of view most seeds only need water and air to germinate but if you are looking to get the best results they should have at least eight to ten hours of light and as much as eighteen to twenty hours of light from the moment they are sown.