What is the meaning of ‘lock down’ here?

"Lock Down" here means to secure something - more specifically, to claim it. The tweet refers to French maids, French kissing, and French braids - that if we call all of those things French, the French must be wild indeed. Then, it mentions French toast, which is good but much less exciting

Okay there are layers to this. It seems like Spanish is your first language. So in spanish you don't really have a word for "lock". It's not Cerrar but it's not Bloquear. A lock, noun, is "la cerradura". So the verb "to lock" means something like "para garantizar que esté bien cerrado con la cerradura" Hahaha When you lock something down you have the key to the lock. It's yours and no one else can claim it. French Maid is a term that exists. French braids are a term that exists. French kissing is a term that exists. And they make a joke about French Toast. And French maids, French braids, and French kissing all also have a sexual context in a way. So this person is saying they were so good at sex that they claimed these words. It's a joke

They're making a joke about how the French have French maids, French braids, and French kissing all named after them, so they must be pretty freaky or kinky. 

"lock down" in this context of means to own or acquire. For example, if a man is dating a woman who is more attractive than he is, you might say "he locked that down" when he marries her.

It's a joke. There's French maids, French braids, and French kissing - all very sexy things. And then there's French toast, which isn't sexy at all.

own, claim ownership of