Remember to be sure your tongue is touching your top teeth for the first sound /ð/. The really tricky bit here, though, is the three consonants /tld/ together. You must pronounce these without a vowel sound coming between them. When you make the /t/ sound, don’t remove your tongue from the top of your mouth. Keep the front of your tongue in place, but lift the middle slightly, letting air pass round the sides (This is much easier than it sounds) and introduce your voice. The result is you move smoothly from /t/ to /l/. Now push the middle of your tongue back up a little to where it was, and you’ll go to the /d/ sound. This will all take some practice, but the important thing is to keep the front of your tongue touching the top of your mouth all the time.
The phrase means “That’s good enough”, or sometimes, “That’s enough”.
However, if you place a lot more stress, and bigger intonation on “do”, it means “Stop it!”, in an imperative way: