Translation:The weather is like that.
Both sound like "shi" to an English ear (though technically the position of the tongue is a bit more to the front. In IPA that sound is written ɕ). The difference between ㅅ and ㅆ is that the latter – like all doubled consonants – represents a "tense" consonant. Instruction books differ somewhat on what "tense" concretely means, my guess is that's because there is more than one way to produce them. Personally, when I say 씨 (or 쓰 or another syllable involving ㅆ), I pronounce a so called glottal stop (the click sound you have in the middle of "uh-oh") at the same time. ㅅ on the other hand typically has a distinct aspiration (h-sound) after it, so 사랑 sounds like s+harang.
Also syllables which start in a tense sound also tend to have a higher pitch, but unfortunately that doesn't help with ㅆ vs ㅅ because ㅅ also causes a fairly high pitch.
My undeestanding is that the sounds are very similar. Both pronounced "shi" but aspirated slightly differently. A Korean coworker tried to explain the difference to me and I could not hear it in the first sitting. I think it would take more immersion to fully grasp it.
Anyone correct or confirm this for me here?