Meaning of ssl_ciphers line on nginx.conf

HIGH, LOW, EXPORT etc are kind of macros which include a range of ciphers. To get the actual ciphers use openssl ciphers command, i.e.

 $ openssl ciphers -V 'HIGH:!aNULL:!MD5'
      0xC0,0x30 - ECDHE-RSA-AES256-GCM-SHA384 TLSv1.2 Kx=ECDH     Au=RSA  Enc=AESGCM(256) Mac=AEAD
      0xC0,0x2C - ECDHE-ECDSA-AES256-GCM-SHA384 TLSv1.2 Kx=ECDH     Au=ECDSA Enc=AESGCM(256) Mac=AEAD
      0xC0,0x28 - ECDHE-RSA-AES256-SHA384 TLSv1.2 Kx=ECDH     Au=RSA  Enc=AES(256)  Mac=SHA384
      0xC0,0x24 - ECDHE-ECDSA-AES256-SHA384 TLSv1.2 Kx=ECDH     Au=ECDSA Enc=AES(256)  Mac=SHA384
      ....

Since the contents of these macros differs between OpenSSL versions you should run this command with the OpenSSL on the servers system. For more details about the ciphers and macros see the man page of the ciphers command.