The house of Israel is the name for the ten northern tribes. Tribes that are now lost in the nations: they left the land of Israel, intermarried with other cultures and basically disappeared without a trace. However they are not lost to YHWH. He knows where his people are and promises that one day he will restore all twelve tribes together.
Since these people are not lost to YHWH, how are they considered lost? It's because the world and the people themselves have no idea that YHWH longs for them to turn to him and come back into his kingdom. Most people in our culture are lost. They've lost their bearings. They've lost the compass of YHWH's instructions as directions to live by. They have no idea that it's likely they have physical ancestry in the people who spent forty years wandering the desert. They have not been taught how to follow YHWH. He longs for individuals worldwide to turn back and become part of his people. He wants them back.
Do you see the implications that this verse gives? Since all who turn to YHWH and serve him get grafted into Israel (whether or not they have family lineage there), that means that you get to pray this. That Israel would grow and grow like crazy.
YHWH has given us the gift of this prayer. Something he will bring about, we can be part of by praying for him to bring the lost sheep of Israel back. More and more people will become part of the community that serves YHWH, keeps his feasts, sets apart the Sabbath day, follows his instructions, and glorifies the life and death of Yahusha. He will bring them back.
Increase our numbers like sheep.