Will the LimeSDR TX at HF frequencies ?
Yes in hardware this should not be a problem, but you will still need an appropriate antenna for the frequency that you wish to transmit (with a good VSWR). At the moment, as far as I’m aware, this option is not available, but software is being written right now to tune the configurable parameters within the board to transmit (and receive) at low frequencies.
What does 10dBm mean ?
10dBm is the power ratio in decibels (dB) of the measured power when referenced to one milliwatt (1 mW). Put “dBm to Watt calculator” into your search engine of choice, if Watt is a more familiar measurement of power to you.
I like http://www.minicircuits.com/pages/pdfs/dg03-110.pdf
(You should probably watch Michael Ossmann excellent video to learn more about dB, dB’s are very useful. Software Defined Radio with HackRF, Lesson 3 - Great Scott Gadgets )
10dBm is 10mW, and you are probably thinking that is a bit low, but people have been able to send their callsign over 2299 km in the 20 meter band using a 10 meter center fed dipole antenna in their attic and only 15dBm of power with no external amplifier. ( https://twitter.com/argilo/status/501072471496601600 https://twitter.com/f1smv/status/501701069857386496 ). Ok, WSPR is a special digital mode where you are only sending 50 bits which are encoded with lots of error correction and transmitted over a 2 minute timeframe 
, but it still counts.
Almost all (general purpose) SDR hardware sold, which can TX, does so at such low power levels. This is so that they qualify as test equipment and fully comply with “FCC 15.103 Exempted devices” under section C “(c) A digital device used exclusively as industrial, commercial, or medical test equipment.” Otherwise, because of their extremely large frequency range, which cover bands that it is illegal to transmit on, the hardware could not be legally sold, at least not in the US and probably a good few other countries as well.
But if you are licensed radio amateur there is nothing stopping you, other than common sense, from adding amplifiers and filters to bring this up to the maximum legally allowed (1,500 Watts 61.76dBm is the peak legal output power for a fully licensed ham radio station in the US/200 Watts 53dBm Peak envelope power for Technicians in the US).