Grafting is taking 2 related plant species and making them grow as 1. Think of a hand transplant. You would be the rootstock and the new hand would be the scion. The scion is basically placed on the rootstock and stitched by mother nature the same as the Dr stitches the hand onto a new body.

Lemons and oranges will give fruit when they have grown to maturity whether they have been grown from seed or grafted onto a rootstock. The main reason citrus is grafted is that:
1) This imparts dwarfing capabilities thus keeping the plant at a smaller more manageable size. Citrus on its own roots can grow to a HUGE size.

2) Some species of citrus do not do that well on their own roots. This means they are more susceptible to root borne diseases than others. When they are grafted onto a rootstock that is able to somewhat withstand these diseases/viruses the scion is able to be kept healthier.

3) Citrus on their own roots do not usually like to be kept in containers. Grafting onto a very slow growing rootstock allows the scion to thrive in a container for those of us who live in the colder "no grow" zones.

Sometimes when a grafted plant is mentioned, the one mentioning does not fully understand the full concept of the grafting process. Usually grafting is done to join a MATURE scion to a juvenile rootstock. With the mature scion you will receive fruit. IF you were to graft a juvenile scion to a rootstock(mature or juvenile) you would NOT receive fruit until the scion was mature enough. This could be many, many years depending on the species.

IF your lemon tree is blooming its mature enough to fruit.
Hopefully this answers your questions.
Andi