$this->prefixLengthsPsr4[$prefix[0]][$prefix] = $length What does it mean?

I don’t know much about psr0 psr4
This is a sentence in composer,

<code>$this->prefixLengthsPsr4[$prefix[0]][$prefix] = $length</code>

Excuse me, what does this sentence mean?

Reply content:

I don’t know much about psr0 psr4

This is a sentence in composer,

<code>$this->prefixLengthsPsr4[$prefix[0]][$prefix] = $length</code>

Excuse me, what does this sentence mean?