Understanding the definition of a language
The expression &quot;$a_i = a_{i+1}$&quot; means &quot;$a_i$ and $a_{i+1}$ are equal&quot;.
Presumably $a_i$ is a symbol, that is, $a_i \in \{0,1\}$. Therefore the condition states that $a$ contains at least four symbols, and there are two adjacent symbols which are equal, that is $a$ contains either $00$ or $11$ as a substring. Stated differently, $a$ contains at least four symbols and it doesn't consist of alternating zeroes and ones (i.e. you are not allowing 0101, 01010, ... or 1010, 10101, ...).