Abarim Publications' online Biblical Hebrew Dictionary
The verb יסף (yasap) means to add, increase or do again. This verb is the common word to use when something is simply added, such as a son for Rachel (Genesis 30:24; hence the name Joseph), or a fifth to a certain value (Leviticus 27:13), or fifteen years to a life (Isaiah 38:5).
But it's also quite commonly used to mean repeat, such as marrying again (Genesis 25:1), or doing something on top of something else, "also," such as hardening one's heart on top of sinning again (Exodus 9:34), or praise some and then praise some more (Psalm 71:14).
Sometimes this verb used to indicate a gradual increase, such as people to a nation (Exodus 1:10) but sometimes the text treats increase as an autonomous concept. This doesn't occur in English, so translating this curious phenomenon takes a trick or two. In Psalm 115:14 for instance, Israel is wished increase, "you and your children".