How to pronounce IPA "/rɑːp/"? (Old English)

Old English had short and long vowels, which were pronounced more or less the same, except the long vowels were longer (in time). In IPA, the /ː/ is a long symbol, which means that the vowel before it is long; so /ɑː/ means the long version of the vowel, while /ɑ/ would mean a shorter version.

For example, hæt, pronounced /xæt/, in Old English had a short vowel, which turned into our modern English hat, with a "short a", and wæt, pronounced /wæːt/ and meaning wet, had a long vowel. The terms "short vowel" (for the vowel in hat) and "long vowel" (for the vowel in hate) derive from the fact that in Early Middle English, these actually were the short and long versions of the same vowel; the vowels have changed since then.

It is pronounced as a long a: /ɑː/ and appears in British English in park, hark, and carp. In some forms of American English, the 'r' may be lightly pronounced where it would not be in /rɑːp/.

To hear "carp" spoken in various varieties of English: https://www.wordreference.com/definition/carp

The sign that looks like a colon in your question should be two small triangles one above the other. It indicates a long vowel. You can see the character here Triangular colon

Listen to the pronunciation of non-rhotic apart in the following video. This will indicate how to pronounce /a:/ - Note that the speaker is Australian but this doesn't affect that particular vowel much.

Vowel Sounds In English With IPA - The Ultimate Video To Improve Your Pronunciation Of Vowels

It's probably worth reading the rest of the article but here's an excerpt:

English Vowel IPA Examples – Long Single Vowels

Vowel Phonetic Symbol & IPA Examples in Words

/i:/ week /wi:k/, feet /fi:t/, media /ˈmiː.di.jə/ /ɑ:/ hard /ha:/, park /pa:k/, article /ɑː.tɪ.kəl/ /ɔ:/ fork /fɔ:k/, walk /wɔ:k/, August /ɔːˈɡʌst/ /ɜ:/ heard /hɜ:d/, word /wɜ:d/, surface /ˈsɜː.fɪs/ /u:/ boot /bu:t/, group /gru:p/, beautiful /ˈbjuː.tɪ.fəl/

What are English Long Vowels in the IPA? There are 5 IPA symbols for English long vowels. The IPA for English long vowels are: /i:/, /ɑ:/, /ɔ:/, /ɜ:/, /u:/.

IPA ENGLISH VOWEL SOUNDS & EXAMPLES

Conclusion

If you are English then you will be used to not pronouncing the "r" in "harp" so in that case , /rɑːp/ would rhyme with "harp".

If on the other hand you are from a country where the "r" in "harp" is pronounced, this may not be so useful.

However /rɑːp/ is Old English so the initial "r" would be pronounced quite strongly.

I hope this helps somewhat. Please feel free to ask further questions.