Bimodal bilingualism means that someone knows two different languages and can use both at the same time. But what makes it different is that one of the languages is not spoken, it is sign language, which uses hand signs to communicate instead of talking.
So, let's say there is a person who speaks English and also knows American Sign Language (ASL). They can use both languages at the same time to communicate with others. They might speak in English while also signing in ASL, or they might sign in ASL while also speaking in English.
This type of bilingualism is called "bimodal" because it uses two different modes of communication - speaking and signing. It's like having two different sets of tools to communicate with, like using both a hammer and a screwdriver to fix something.
Not everyone who is bilingual is bimodal, and not everyone who knows sign language is bilingual. But for those who are bimodal, it can be a really helpful skill when communicating with other people who use sign language, or when speaking with someone who might not understand spoken language very well.