I would say it is not totally clear for me neither

Samtools definition of TLEN :

If all segments are mapped to the same reference, the unsigned observed template length equals the number of bases from the leftmost mapped base to the rightmost mapped base.

--no-templatelen-adjustment

Disables template length adjustment for RNA-seq reads.

In your case as you're working with single end, I think that the size of the template is equal to the size of your read

I do not know the effects of disabling the template length adjustment

But in any case if you don't understand an option, do not use it

Some info : https://github.com/infphilo/hisat2/issues/116

Edit :

Template length adjustment for RNA-seq reads is used to fit TLEN to the transcript length template due to splicing events

Using the --no-templatelen-adjustment you will got a template length based on the reference sequence (which in not what you want in typical RNA-Seq analysis).

But some tools as Manta (issue above) needs this template length information to the reference.

In conclusion, do not use it