In the Dante's Inferno canto I, it speaks about "those who are happy and in fire." What does that mean?
Virgil is speaking to Dante in the lines in question, 1.1.118-120. To understand these lines, it is useful to back up a bit, to the previous six lines.
At the end of the first Canto, Virgil is preparing his acolyte for the levels of hell he will witness. The first being the "eternal place,/where you shall hear the howls of desperation/and see the ancient spirits in their pain, as each laments his second death."
The following lines, "and you shall see those souls who are content within the fire, for they hope to reach -- wherever that may be -- the blessed people." Unlike the first set, whose eternal doom appears to be sealed, these souls seem to have some hope of eventual escape from their torment and are biding their time while understanding that some penance in hell must be paid for their earthly sins.