Literally, they believe there is nothing He cannot do.
Although there is no evidence that this is true, this belief is based on certain quotes in the Bible.
In particular, Jesus is quoted as saying
"with God all things are possible" (Matthew 19:26). This quote suggests Jesus thought that nothing was impossible for God. This brings up the question: can God break his own physical laws to bring about a certain outcome? Only those who have experienced a direct breaking of a physical law can speak on this matter. Another question is: does God only work in the universe through beings that are present in this universe or can God act without the intermediary of such beings?
Just because one believes that one or more Gods created this universe, it doesn't necessarily follow that this God or these Gods are all powerful. This is purely speculation on man's behalf. Because Christians view Jesus as all knowing (although there is certainly no proof of this- it's just an assumption) they take his words as being completely true and beyond question. Therefore, Christians don't question his statement as to the unlimited power of God.
This belief by Christians in God's omnipotence makes it hard for many of them to understand why there is suffering in the world. If God is all powerful why doesn't he end all suffering? The obvious answer is that God sees suffering as serving a purpose that would be thwarted if the suffering were to be ended by an external agency. In this case, suffering would be seen as serving a greater purpose than would be served if people just felt good no matter what they did. Therefore, the existence of suffering really says nothing about whether God is all powerful or not.
There is actually no way to know if a creator God is all powerful without seeing such a God in action. Clearly, the existence of such a mind-blowingly massive, complex and beautiful universe suggests that any being(s) that created it must have creative powers that are beyond our imagination. But this alone does not tell us whether these powers are unlimited, even in regards to the manipulation of their own creation, once it has been set in motion.