Islam teaches that each person is born a Muslim. For some, due to circumstances – namely, the corruption of the child’s parents – the child grows up in a false religion. Thus, when a non-Muslim converts to Islam, they say he/she is reverting to Islam. In other words, the person is returning to his/her original religion, his/her religion of birth (literally).
Islam teaches that upon conversion, a person’s sins are forgiven. It is as if he/she is reborn.
Quite often, conversion occurs for marriage. That is, a non-Muslim wants to marry a Muslim. Although a Muslim man may marry certain non-Muslim women, it is still considered expected that the woman will convert. Because a Muslim woman is forbidden to marry a non-Muslim, the non-Muslim man would have to convert to Islam for the woman to still be considered a Muslim. (If a non-Muslim woman married to a non-Muslim man converts to Islam, she is considered to be un-married, and would have to have her husband convert and then re-marry him or risk becoming a non-Muslim.)
Conversion to Islam is considered a person’s most important decision, and thus should not be taken seriously. Being an infidel is not as bad as being an apostate. Many hold that belief in Islam is a prerequisite for entering Heaven. No infidel will enter Heaven, so conversion to Islam is a matter of spiritual life or death.