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tasya lopaH तस्य लोपः ONPANINI 13009

Labelletters poof.

A clearer translation would be —

Some of the letters that appear in grammar teaching are mere labels and never make it into an actual word.

These label letters are called "/it" (fake letters) by /pANini.

Example.

sUtra GyApprAtipad... teaches, among other things, "add /su after **azva- अश्वॱ when this and that happens". Using that sUtra and others we build the sentence —

dru द्रु + /laT + **azva- अश्वॱ + /su + pause
→ dravati द्रवति + azva- अश्वॱ + s स् by this rule
→ dravati द्रवति + azva- अश्वॱ + H ः by several rules
→ dravatyazvaH द्रवत्यश्वः

In the word dravati द्रवति too, two p प् and a z श् have disappeared, because that word was formed like this —

dru द्रु + /laT /tip → dru द्रु + /zap + /tip → dro द्रो + /zap + /tip by gbH → drav द्रव् + /zap + /tip → dravzaptip द्रव्शप्तिप् → **dravati द्रवति by this rule

Why does the p प् in /tip disappear?

Why does the u उ in /su disappear?

Long answer — the u उ disappears because the word "su सु" is the name of a particular kind of s स्, namely, the s स् that must be added after nounbases when sUtras GyApprAtipad... and svaujasamauTch... and others teach so. There is another word "/sic" that means "a certain s स् that is added to some verbs", and a word "sa स" that means "any s स् whatsoever, including, but not limited to, /su and /sic".

Short answer — the u उ disappears because teacher told you that u उ is a labelletter, and this rule teaches that labelletters disappear.

See also the page questions about tasyalopaH.