What are "split reads" and "intron clusters?"
split reads - These are read that have two or more alignments to the reference from unique region of the read.  In this example a 150bp read sequenced from RNA could have base 1-75 aligning to the 3' end of exon2 and bases 76-150 aligning to the 5' end of exon3.  This would be a split read because it have two alignments (exon2 and exon3) and those alignment are from unique (non overlapping) parts of the read, 1-75 and 76-150.

Chimeric reads are an example of split reads.  Many people use "chimeric reads" to describe a type of noise that occur during PCR or sequencing where the bases from two different molecules are reported as one read and the alignments of that read become split when it aligns back to the location of the two original molecules.

"intron clusters" - In this case I think that the LeafCutter software is describing stacks of split reads to cluster to support the detection of an intron.