Reading aloud means that someone reads something aloud for other people. The opposite is silent reading. Reading aloud is especially useful for those who cannot read themselves. However, people often find it nice to be read to, even though they could read it themselves.
For a long time it was normal to read aloud. For only a few people could read and write and so the others could listen in. For example, a ruler would send messengers to their country. In the individual villages and towns, they would then read out a message from the ruler.
Today, we are mostly familiar with reading aloud from the radio or television. Many parts are spoken freely there, for example in interviews or as a moderation to music. However, the messages are always written beforehand and then read out.