A linguistic universal means that there are certain things that are common to all languages no matter where they're from or who speaks them. For example, all languages have nouns and verbs. Nouns are words that describe things like people, places or things while verbs are words that describe actions like walking, talking or eating.
Another universal in language is that they all have grammar rules. This means that all languages have a certain way of organizing their words to form sentences. So for example, in English, we say "I am walking to the store." But in another language, the sentence structure might be a bit different.
There are also certain sounds or speech patterns that are common across all languages. For example, all languages use vowels and consonants to form words.
Overall, a linguistic universal means that there are certain things that are true for all languages no matter where they come from or who speaks them.