The term "gateway drug" is used to describe any drug that a non drug-user might try out. Often a gateway drug is not perceived to be as potent, harmful or addictive, giving the potential user the feeling that "it can't hurt me". The psychology of such behavior is that once you try something like that, you tend to want more, and since gateway drugs lower inhibitions just like their more powerful addictive drugs, often people will try "something else" while they are in the middle of experimentation with gateway drugs.
The term is used because the drugs act as a "gateway" to the use of other drugs.

I got this from another website. -----^
Weed is often called a gateway drug. The danger most people associate with smoking weed is that it will lead to stronger drugs. The reason behind this idea is that once your body reaches a level of tolerance for weed it needs the next best thing to reach your desired high. Personally I think its true but I also believe it depends on the individual user. Look at snoop that fool been smoking since the 80’s and theres nothing wrong with him. His mind is still functioning properly, how else can you explain someone making millions a year even though he hasn’t dropped a good abulm since doggy style. I don’t think theres dangerous damage from the actual use of weed.

I agree that people that actually need it should be allowed to smoke it.

But, people always try to take advantage of everything. Point being is that ive seen regular people getting medical weed cards from worthless doctors just to make money. When you get the card the government allows you to grow up to 5 plants in your house, so if you don’t smoke weed and get a card you can sell the weed for extra cash. To get a card in CA all you need to say is you suffer from migraines. Dam this headache is killing me.