ככל שאני מתרגלת בתדירות גבוהה יותר, כך הניידות שלי משתפרת.

It is a very common Hebrew pattern meaning the more ..., the more ... or the more ..., the better ....

So here:

• ככל שאני מתרגלת בתדירות גבוהה יותר = the more frequently I practice
• כך הניידות שלי משתפרת = the more my mobility improves / my mobility improves accordingly

In other words, it expresses a relationship between two changing things.

In Hebrew present tense, the verb usually does not show person clearly the way English does. It mainly shows gender and number.

So מתרגלת tells you the subject is feminine singular, but by itself it could mean something like:

• I practice (if the speaker is female)
• you practice (addressing one female)
• she practices

That is why אני is needed here: it makes it clear that the subject is I.

Because the speaker is feminine.

• מתרגלת = feminine singular
• מתרגל = masculine singular

So a male speaker would say:

ככל שאני מתרגל בתדירות גבוהה יותר, כך הניידות שלי משתפרת.

Only the forms that agree with the speaker or subject change.

It is the Hebrew present form, but Hebrew present tense is broader than English.

מתרגלת can mean:

• I practice
• I am practicing
• I do practice

In this sentence, it has a habitual/general meaning, so in natural English it is understood as I practice or I practice more frequently, not necessarily something happening only at this exact moment.

Literally, it means at a higher frequency.

Breakdown:

• ב־ = in / at / with
• תדירות = frequency
• גבוהה = high (feminine singular)
• יותר = more

So the whole phrase means more frequently.

This is a somewhat formal or precise way to say it. In English, we would usually turn it into the adverb more frequently.

Because it describes תדירות, and תדירות is a feminine noun.

In Hebrew, adjectives must agree with the nouns they describe in gender and number.

So:

• תדירות גבוהה = high frequency
• not תדירות גבוה

That is why the feminine form גבוהה is used.

You could say אני מתרגלת יותר, but it is a little less specific.

• אני מתרגלת יותר = I practice more
• אני מתרגלת בתדירות גבוהה יותר = I practice more frequently

The first one could mean more overall or for a longer amount of time.
The second one specifically focuses on how often the practice happens.

So the original sentence sounds more precise and slightly more formal.

ניידות means mobility.

The ה־ is the definite article, so הניידות literally means the mobility.

In Hebrew, when a noun is followed by שלי to mean possession, it is very common for the noun to be definite:

• הניידות שלי = literally the mobility of mine
• natural English: my mobility

So the ה־ is normal here.

Modern Hebrew usually expresses this kind of possession with:

noun + possessive pronoun

So:

• הניידות שלי = my mobility
• הספר שלי = my book
• הבית שלי = my house

English puts the possessive word before the noun, but Hebrew very often puts the possessive element after the noun.

So even though it looks more like the mobility of mine, it simply means my mobility.

Because משתפרת means improves / gets better, while משפרת usually means improves something.

Compare:

• להשתפר → to improve / get better
  → הניידות שלי משתפרת = my mobility is improving
• לשפר → to improve something
  → אני משפרת את הניידות שלי = I improve my mobility

In the sentence you gave, mobility is the thing that is changing by itself, so משתפרת is the correct choice.

Also, it is feminine singular because ניידות is feminine singular.

In standard Hebrew, ככל ... כך ... is the full and most natural pattern.

So this is the clearest and most complete version:

ככל שאני מתרגלת בתדירות גבוהה יותר, כך הניידות שלי משתפרת.

In less formal speech, people sometimes omit כך if the meaning is obvious, but keeping it is better, especially for learners, because it clearly marks the second half of the the more ..., the more ... structure.