Japanese uses three distinct character sets in writing: Hiragana (a syllabary), Katakana (another syllabary) and Kanji (Chinese characters). Hiragana and Kanji are usually used together to write native Japanese words, whereas Katakana is used almost exclusively for loanwords.

Katakana syllables are either bare vowels, or consonant+vowel (with few exceptions), meaning that transliterated words must be contorted to fit the syllabary (the "lossy algo" referred to by the parent comment). So, something like "kernel" in English becomes kaa-ne-ru, and "subsystem" becomes "sa-bu-shi-su-te-mu".

Katakana [1] is one of the two syllabary writing systems in Japanese, with Hiragana [2] being the other. IIRC katakana is the one where every syllable starts with a consonant and ends with a vowel, so transliterated loan words with consecutive consonants end up growing vowels in the middle. They also have kanji (characters), and various romanization systems (https://en.wikipedia.org/wiki/Romanization_of_Japanese). It's really amazingly inefficient.

[1] https://en.wikipedia.org/wiki/Katakana [2] https://en.wikipedia.org/wiki/Hiragana

Why is everyone posting squares?

The Japanese first adapted writing from Chinese; this adapted system is called kanji and is highly similar to, and often homographic with, Chinese hanzi. However, Japanese and Chinese have very different phonological structure: Chinese is mostly comprised of monosyllabic words, whereas Japanese has lots of polysyllabic words. This makes it more difficult to do phonetic transcription in Japanese kanji than in Chinese hanzi.

The Japanese got around this by simplifying the script into a syllabary (every character represents roughly a syllable, or more often, a consonant-vowel pair). They did this twice: one of these syllabaries is hiragana, and the other is katakana. In modern usage, katakana is used largely for phonetic transcription, such as transliteration of foreign words and names, or onomatopoeia, whereas hiragana is used for writing out Japanese words.

As others have pointed out, Japanese has a rather constrained phonological system, so a word like strengths cannot be represented directly but rather more like "su-to-re-n-ge-tsu." Of course, this feature isn't limited to Japanese; it's how an island called "Christmas" gets transliterated to "Kiritimati", just like its parent "Kiribati" is the local pronunciation of "Gilbert." While people think it's annoying, it's largely because they haven't faced languages with challenging transciptions into Latin script. There's a reason why there's a plethora of transcriptions of "مُحمّد‎" after all.